SYNOPSIS

DESCRIPTION

The system provides each process with three interval timers,
each decrementing in a distinct time domain.
When any timer expires, a signal is sent to the
process, and the timer (potentially) restarts.

ITIMER_REAL

decrements in real time, and delivers
SIGALRM
upon expiration.

ITIMER_VIRTUAL

decrements only when the process is executing, and delivers
SIGVTALRM
upon expiration.

ITIMER_PROF

decrements both when the process executes and when the system is executing
on behalf of the process.
Coupled with
ITIMER_VIRTUAL,
this timer is usually used to profile the time spent by the
application in user and kernel space.
SIGPROF
is delivered upon expiration.

The function
getitimer()
fills the structure pointed to by
curr_value
with the current setting for the timer specified by
which
(one of
ITIMER_REAL,
ITIMER_VIRTUAL,
or
ITIMER_PROF).
The element
it_value
is set to the amount of time remaining on the timer, or zero if the timer
is disabled.
Similarly,
it_interval
is set to the reset value.

The function
setitimer()
sets the specified timer to the value in
new_value.
If
old_value
is non-NULL, the old value of the timer is stored there.

Timers decrement from
it_value
to zero, generate a signal, and reset to
it_interval.
A timer which is set to zero
(it_value
is zero or the timer expires and
it_interval
is zero) stops.

Both
tv_sec
and
tv_usec
are significant in determining the duration of a timer.

Timers will never expire before the requested time,
but may expire some (short) time afterwards, which depends
on the system timer resolution and on the system load; see
time(7).
(But see BUGS below.)
Upon expiration, a signal will be generated and the timer reset.
If the timer expires while the process is active (always true for
ITIMER_VIRTUAL)
the signal will be delivered immediately when generated.
Otherwise the
delivery will be offset by a small time dependent on the system loading.

RETURN VALUE

On success, zero is returned.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EFAULT

new_value,
old_value,
or
curr_value
is not valid a pointer.

EINVAL

which
is not one of
ITIMER_REAL,
ITIMER_VIRTUAL,
or
ITIMER_PROF;
or (since Linux 2.6.22) one of the
tv_usec
fields in the structure pointed to by
new_value
contains a value outside the range 0 to 999999.

NOTES

A child created via
fork(2)
does not inherit its parent's interval timers.
Interval timers are preserved across an
execve(2).

POSIX.1 leaves the
interaction between
setitimer()
and the three interfaces
alarm(2),
sleep(3),
and
usleep(3)
unspecified.

BUGS

The generation and delivery of a signal are distinct, and
only one instance of each of the signals listed above may be pending
for a process.
Under very heavy loading, an
ITIMER_REAL
timer may expire before the signal from a previous expiration
has been delivered.
The second signal in such an event will be lost.

On Linux kernels before 2.6.16, timer values are represented in jiffies.
If a request is made set a timer with a value whose jiffies
representation exceeds
MAX_SEC_IN_JIFFIES
(defined in
include/linux/jiffies.h),
then the timer is silently truncated to this ceiling value.
On Linux/i386 (where, since Linux 2.6.13,
the default jiffy is 0.004 seconds),
this means that the ceiling value for a timer is
approximately 99.42 days.
Since Linux 2.6.16,
the kernel uses a different internal representation for times,
and this ceiling is removed.

On certain systems (including i386),
Linux kernels before version 2.6.12 have a bug which will produce
premature timer expirations of up to one jiffy under some circumstances.
This bug is fixed in kernel 2.6.12.

POSIX.1-2001 says that
setitimer()
should fail if a
tv_usec
value is specified that is outside of the range 0 to 999999.
However, in kernels up to and including 2.6.21,
Linux does not give an error, but instead silently
adjusts the corresponding seconds value for the timer.
From kernel 2.6.22 onwards,
this non-conformance has been repaired:
an improper
tv_usec
value results in an
EINVAL
error.